Field of the Invention
The instant invention relates to methods and apparatus to improve the dynamic coherent length of a sweep velocity-locked laser pulse generator in an all-electronic fashion. The enhanced laser coherence, or coherent length, can extend the measurement range for applications including frequency-modulated continuous wave (FMCW) light detection and ranging (LiDAR) and optical fiber distributed sensing applications.
Description of Related Art
At the core of both FMCW LiDAR and OFDR distributed sensing technology is a coherent optical source, or laser, that can output linearly swept frequency as a function of time. Currently, one cost-effective solution to achieve a frequency-swept laser is through modulating the injection current of the semiconductor laser. However, the corresponding frequency sweep is not sufficiently linear as a function of time, resulting in distorted measurements in the Fourier Transform domain. Previously, a frequency/wavelength calibration method comprising the introduction of an interferometer with a long and fixed delay line was used to correct the non-linearity of swept-laser source. However, this method required the use of a very long delay line, resulting in high vibrational noise and a bulky design. Another solution was through the use of an optical phase locked loop (OPLL) architecture to lock the sweep velocity to a stable reference source within the entire length of laser pulse duration.
However, there remains a critical issue, which is the broad linewidth of the semiconductor lasers. For example, the distributed feedback lasers (DFB) typically hold a linewidth of around a few MHz, resulting in a maximum measurement range of only a few tens of meters for both FMCW LiDAR and OFDR distributed sensing applications. Cost-effective semiconductor lasers, such vertical emitting cavity surface laser (VECSEL), have broader linewidth and shorter measurement range. In order to reduce the linewidth, or improve the coherent length, of the swept semiconductor lasers, an acoustic optical modulator (AOM) was employed in a servo-loop to dynamically mitigate the optical phase noise at the output of the semiconductor laser, mitigating the phase noise of semiconductor laser with a high control loop bandwidth, and resulting in a much narrower linewidth. However, the use of AOM is associated with a high driving voltage, high-cost, and bulky design.